1. Field of the Invention
This invention relates to a compact behind stop modified TESSAR.RTM. lens having an angle of view of 60.degree. or more and brightness of the order of F2.8 and comprising three groups of four lenses.
2. Description of the Prior Art
The TESSAR.RTM. lens has a feature that the full length of the lens is relatively short and the aberration fluctuation by the object distance is relatively small, and is known as a compact optical system. Particularly, the behind stop TESSAR.RTM. lens enables the distance adjustment to be accomplished with the stop being fixed and by moving only the lens forwardly and therefore, it is best suited for the mechanism of a compact auto focus camera and has often been put into practical use.
In the usual TESSAR.RTM. lens, a third component has a cemented surface convex relative to the object side, but numerous modified TESSAR.RTM. lenses having a cemented surface concave relative to the object side are known. In such modified TESSAR.RTM. lenses, the sagittal image plane is somewhat flat and the full length of the lens can be made somewhat shorter while maintaining the balance of various aberrations. Now, in the case of a behind stop lens, of the oblique light beam, the light rays passing through the marginal portion of a first lens pass through a portion near the center of the stop and therefore, a decrease in marginal light is liable to be conspicuous, and this leads to a disadvantage that the deficiency of the quantity of marginal light is difficult to correct even if stopped down. In the TESSAR.RTM. lens, it is possible to increase the effective diameter of a first lens and considerably reduce the decrease in marginal light, but occurrence of coma and chromatic coma is conspicuous, and particularly in a modified TESSAR.RTM. lens having a cemented surface concave relative to the object side, occurrence of chromatic coma has been substantial.